Question

What could possibly be the problem if a valid function name such as tolower() is being reported by the C compiler as undefined?

Answer 1

The most probable reason behind this error is that the header file for that function was not indicated at the top of the program. Header files contain the definition and prototype for functions and commands used in a C program. In the case of “tolower()”, the code “#include <ctype.h>” must be present at the beginning of the program.